Thermal printers create images on a thermal sensitive medium when the medium passes adjacent to a local hot spot on a thermal printhead. In direct thermal printers, the medium is a temperature-sensitive paper which passes by in intimate contact with an electrically resistive print bead on the printhead. In thermal transfer printers, the image is transferred from a thermal transfer ribbon carrying a waxy ink to the print stock, the thermal transfer ribbon passing by in intimate contact with the thermal printhead. For both printers, passage of the temperature-sensitive paper or the thermal transfer ribbon past the printhead causes a separation of charge resulting from the triboelectric effect. This effect generates a static electric charge on the surface of the printhead. Thermal printheads are generally made from electrically nonconductive material so that the static charge generated by the triboelectric effect tends to stay where it is.
The build-up of static charge on the surface of a thermal printhead is a primary cause of printhead failure. Studies indicate that the accumulation of charge near the print bead of the printhead leads to a breakdown of local areas of the print bead, causing the local resistance of the print bead to diminish. If the resistance is diminished sufficiently to cause a significant rise in power dissipation, this initial resistance change alone, i.e., the resistance decrease due to static discharge, is sufficient to be considered a failure, especially in applications where it is important to control the size of a printed element, such as bar code. Additionally, the localized areas having diminished resistance experience wider temperature extremes that cause those areas of the print bead to undergo more thermal stress than the remainder of the print bead.
In the case where the initial resistance decrease due to static discharge is not sufficient to be considered a failure, ultimate failure may occur after the added thermal stress causes the localized area of the print bead to undergo an irreversible resistance increase to the point where the local area is incapable of heating the print medium sufficiently to create a mark.
The need to control static electricity has long been recognized in environments requiring the passage of sheets of paper and the like over various conveyor systems. Anti-static agents, which are generally water-absorbent, can be added to the stock when it is manufactured. However, such agents have been found completely effective only at relative humidities above 40%. Below 20% relative humidity, these anti-static agents have not been found to add significantly to the printhead life. Similarly, passive static dissipation devices, such as grounded carbon fiber or copper tinsel, can be used to generate ions, but not sufficiently near the area where the static charge separation is taking place. Active systems, such as electrical or nuclear ion generators, may be effective at dissipating static charge at the printhead, but are prohibitively expensive for this application.